1. Field of the Invention
The invention pertains to object identification systems and more particularly to a novel system for identifying objects with coded labels using a continuous wave radio frequency roll-call technique.
2. Description of the Prior Art
The label identification systems presently in use are subject to a number of limitations. The systems employing optical techniques are adversely affected by environmental limitations including deterioration of the label and interference of operation in bad weather conditions. A typical optical system is described in U.S. Pat. No. 3,744,026 entitled "Optical Label Scanning". The operation of this system and other optical systems is further limited in that the cooperating objects are required to move in a uniform single file procession over a narrowly-defined path past a scanner.
Prior art label identification systems employing RF techniques have overcome the limitations of the optical systems due to environment. These systems illuminate the cooperating labelled object and receive from the labelled object a complex signal containing a label identification. Due to the complex nature of this signal received from the labels by the interrogating units, these systems, similar to the optical systems, are limited by the requirement that the cooperating labelled objects proceed past the interrogator in an uniform single file procession and further limited by the complexity of the interrogators necessary to decode the signals received from the labels. One such system is described in U.S. Pat. No. 3,247,508 entitled "Microwave Identification of Railroad Cars". In this system, railroad cars have affixed thereto a coded reflector plate into which are cut a series of dipole slots disposed at angles of .+-.45.degree. from a vertical axis. These slots are illuminated with a microwave signal and reflect back to a receiving antenna at the interrogator a series of polarized microwave signals which form the code for each label.
Another such RF identification system is described in U.S. Pat. No. 3,209,350 entitled "Identification Interrogation System". In this sytem, the device to be identified consists of a plurality of piezoelectric elements having different preselected frequency responses. As each such identification device passes an interrogator, signals are transmitted over a preselected frequency range from the transmitting antenna at the interrogator. Signals at the same frequencies of the piezoelectric elements connected in the identification device are repeated by the identification device, transmitted to the receiving antenna at the interrogator, and decoded to indicate the identity of the object.
Still another system employing an RF identification system is described in U.S. Pat. No. 3,406,391 entitled "Vehicle Identification System". In this system, the objects to be identified have affixed thereto a transponder assembly. As the transponder passes the interrogator, it is illuminated with electromagnetic radiation consisting of a series of discrete carrier frequencies each of which is amplitude modulated by one of a plurality of modulating frequencies. The transponder consists of series of frequency responsive devices which when illuminated by the interrogator will receive the series of carrier frequencies, demodulate each such carrier frequency, and selectively radiate a plurality of the demodulating frequencies in sequential order back to the interrogating unit.
It can therefore be appreciated that these prior art RF identification systems have failed to overcome the limitations enumerated above and are, therefore, limited in their application and limited by their complexity.